It is known to filter signals, such as RF (radio frequency) and/or IF (intermediate frequency) signals in transmitters and/or receivers of wireless communications equipment, using surface wave device resonator filters. It is also known for such resonator filters to provide for conversion between balanced and unbalanced signals, so that the need for a separate balun transformer is avoided.
For example, Saw et al. U.S. Pat. No. 5,365,138 issued Nov. 15, 1994 and entitled "Double Mode Surface Wave Resonators" describes a WCR (waveguide or transversely coupled resonator) filter in which one of two IDTs (interdigital transducers) and its outer rail are divided into two oppositely phased halves for providing a balanced signal connection, and the other IDT provides an unbalanced signal connection, so that the filter provides a balun function. LCR (longitudinally coupled resonator) filters similarly providing a balun function are described for example in an International Patent Application published Jan. 3, 1997, Publication No. WO 97/00556, entitled "Cascaded Surface Wave Device Filters".
The related application referred to above describes and claims surface wave device resonator filters using SPUDTs (single phase unidirectional transducers) or RSPUDTs (resonant SPUDTs). An object of this invention is to provide a surface wave device resonator filter, particularly a SPUDT or RSPUDT resonator filter, with a balun function.